1. Field of the Invention
The present invention relates generally to a liquid crystal display, and more particularly to a liquid crystal display with a diffusion layer for improving gray level inversion.
2. Description of the Related Art
In recent years, a transmissive liquid crystal display has been applied to various uses. In these uses, the liquid crystal display is often viewed not only in a front-face direction (normal direction) but also in various directions (angles). Under the circumstances, there is a demand for a display mode that can realize widening of a viewing angle.
As the display mode of the transmissive liquid crystal display, a TN (Twisted Nematic) mode is most widely used because of its advantages of high definition and high transmittance. In the case where gray-level display is performed in the TN mode, gray level inversion takes place when the screen is viewed at a certain angle and the display quality deteriorates.
For example, in the case of a 90° TN mode liquid crystal display that is capable of 64-level gray scale display, 8 levels of the 64 gray levels are chosen and the luminance (cd/cm2) at each gray level is measured at respective observation points at different angles relative to the normal of the screen. This luminance measurement was carried out by measuring, with use of a luminance meter 2, the luminance at observation points with different off-axis angles θ1 toward a major-axis direction D of liquid crystal molecules relative to a normal Z1 of a liquid crystal display 1. In general, the major-axis direction D of liquid crystal molecules is set to be downward of the screen (“downward viewing angle”).
In the TN mode, each gray level is reproduced by controlling the orientation state of liquid crystal molecules by applying an electric field on a pixel-by-pixel basis. In this case, as shown in FIG. 2, when the off-axis angle θ1 is gradually increased toward the major-axis direction D of liquid crystal molecules, that is, toward the lower side of the screen, gray level inversion occurs at a certain off-axis angle.
Specifically, the luminance versus off-axis angle curve associated with each gray level has a minimum value. In addition, since the orientation state of liquid crystal molecules varies from gray level to gray level, the minimum value varies from gray level to gray level. Consequently, at a certain off-axis angle, the luminance of a gray level that is close to black may be higher than the luminance of a gray level that is close to white (“gray level inversion”). As the off-axis angle, at which the gray level inversion occurs, becomes closer to the normal direction (front-face direction of the screen), the screen becomes more unsightly, leading to degradation in image quality.
In order to prevent gray level inversion, wide-viewing-angle liquid crystal displays that adopt an IPS (In-Plane Switching) mode or an MVA (Multi-domain Vertical Alignment) mode have been put to practical use. However, in these liquid crystal displays, the transmittance is low and the drive voltage is high. Since it is difficult to realize low cost and high luminance, there is a demand for improvement of these displays.
On the other hand, there has been proposed an anisotropic light scattering film having light scattering properties that depend on light incidence angles. The light scattering film has such characteristics that the haze value of 80% or more is obtained with respect to light that is incident with an incidence angle in a specified range (e.g. between 0° and 60°) and the haze value of 20% or less is obtained with respect to light that is incident with an incidence angle in a symmetrically opposite range (between −60° and 0°) (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2000-171619).
The above-described light scattering film, however, has such characteristics that a high haze value is obtained with respect to light that is incident with incidence angles in a wide range. Consequently, unnecessary light scattering occurs, which may lead to degradation in transmittance or blurring of a display image. Thus, the display quality may deteriorate.